Wettability Of Ceramics By Molten Rare Earth Metals (La, Ce) And Their Interfacial Microstructures

In recent years, extensive researches on the physical and chemical properties of rareearth （RE） elements have greatly promoted their applications. The RE elements are regardedas the vitamin of metals. Their activity is only inferior to the alkali metals and thealkali-earth metals, suggesting that the addition of a minute amount of the RE elements cangreatly enhance some properties of metals or alloys. The RE elements, which aresurface-active, play an important role in metallurgy of materials such as alloying,purification, and strengthening of alloys as well as improvement in their corrosive resistance.However, only limited work has so far been concerned with the application of REelements in improving the interface or wettability of metal/ceramic systems. Previousresearchers usually add the RE elements to common metals such as Al and Cu to improve thewettability of metal/ceramic systems. However, because of ready oxidation of the REelements and traditional sessile drop method they usesd, an accurate evaluation of the REelements’ role in improving the wettability of metal/ceramic systems becomes rather difficult.In addition, since most of the researchers used binary or multiple alloys as melts, thealloying elements might react with the ceramic substrates, which will also bring somedifficulties to study the RE elements’ role in influencing the wettability. In order tosuccessfully reveal the essential effect of the RE elements on the wettability of alloy/ceramicsystems, the wettability between pure RE metals and ceramics must be understoodcomprehensively. In this dissertation, we selected La and Ce as melts to study the wettingand interfacial microstructures between them and ceramics （oxides, carbides and nitrides）using a modified sessile drop method, expecting to provide useful information and positiveguidance for the preparation of metal/ceramic composites with excellent properties. Themajor results of the present study are as follows:（1） The wetting of molten La and Ce on the oxides （MgO, Al₂O₃, SiO₂and ZrO₂）,nitrides （Si₃N₄and AlN） and carbides （B₄C, SiC, WC, TiC and ZrC） substrates is in the orderof oxides<nitrides<carbides. The strength of chemical bonds in the substrates and thechanges of the interfacial thermodynamic properties determine the apparent wettability. InRE/carbide systems with interfacial reactions, the formation of interfacial products such asRE borides （i.e., RE/B₄C system） and RE carbides determines the final apparent wettability;in RE/nitride systems, although the dissolution of the nitride substrates improves the wettability, the effect is not very significant; whereas, in RE/oxide systems, because of readyoxidation of La and Ce even in a high vacuum, the triple line can be pinned easily by theformation of the RE oxides at the vicinity of the triple line, thus leading to the worstwettability in these systems.（2） The wetting behavior of RE/oxide systems consists of spreading, receding andwarping stages. A mass of RE oxides phase formed at the interface and then grew along theliquid surface, leading to the deterioration in the wettability of the system and finally therecession and warping of the triple line. On the other hand, the gaseous phase （such as Mgand SiO） produced by the interfacial reaction escaped from the drop through evaporation,providing another impetus for the movement of the triple line.（3） From the viewpoint of the apparent contact angle when the liquid reached maximumspreading extent, the apparent wettability of molten La and Ce on the oxide substrates （MgO,Al₂O₃, SiO₂and ZrO₂） is in the order of MgO>Al₂O₃>SiO₂>ZrO₂. The difference inwettability is dictated by the extent of the interfacial reaction as well as by the nature of thereaction product. The transverse growth of the RE oxides along the interface promoted thespreading of the liquid while the longitudinal growth of the RE oxides at the triple linedeteriorated the wettability.（4） The apparent wettability of carbides （B₄C, SiC, WC, TiC and ZrC） substrates bymolten La and Ce is in the order of B₄C>WC>SiC>ZrC>TiC. Generally speaking, thewettability of the systems with interfacial reactions, such as RE/B₄C, RE/SiC and RE/WC, issuperior to the non-reactive systems （e.g., RE/TiC and RE/ZrC）. That is to say, the formationof the interfacial products such as RE borides and RE carbides effectively improves thewettability. For instance, since the RE elements have a particular affinity with boron and theRE borides formed between them （e.g., tetraborides REB4） are metallic-like compounds, theformation of these compounds at the interface greatly improves the wettability in RE/B₄Csystem.（5） The apparent wettability of molten La and Ce on the AlN substrates is slightly betterthan that on the Si₃N₄substrates. The spreading behavior of the RE/nitride systems iscontrolled by the dissolution of nitride substrates.